Exploring the spectral characteristics of the periodic burster 4U 1323-62: Type-I X-ray burst and persistent emission

TL;DR

This study analyzes a ~90 ks NuSTAR observation of the periodic burster 4U 1323-62 to simultaneously characterize persistent emission and six short Type-I X-ray bursts. The persistent spectrum is well described by absorption plus a Fe-like edge at ~7.4 keV and thermal Comptonization with disk-seeded photons, while burst spectra require an edge plus Comptonization and a blackbody component, with time-resolved fits revealing burst-driven enhancements of the persistent emission quantified by the scaling factor $f_a$. Time-resolved spectroscopy shows the blackbody temperature and radius peak near the burst maximum (S2) and that $f_a$ can reach values up to ~6 in the strongest burst, indicating strong burst–disk/corona interaction likely via Poynting–Robertson drag; concurrent hard X-ray deficits support coronal cooling during bursts. The inferred ignition depths of $y_{ m ign} ightarrow ext{(0.6–0.63)} imes 10^8$ g cm$^{-2}$ and α-values in the 54–74 range point to short, mixed Hydrogen/Helium bursts, consistent with a clocked burster behavior and complex accretion-disc response during thermonuclear flashes.

Abstract

We report on the results obtained by the analysis of persistent and type-I thermonuclear X-ray burst emission observed from the periodic burster 4U 1323-62. These analyses are based on the NuSTAR observation performed on 2024 August 7 for a total exposure of around 90 ks. The persistent emission is well described by an absorbed thermal Comptonization model. An absorption edge is also detected at an energy of approximately 7.42 keV, which indicates the presence of absorbing material in the vicinity of this system. Six bursts have been observed during this observation, wherein we find the burst recurrence time to be approximately 4.52 hr. All the bursts exhibit the characteristics of a sharp rise and exponential decay. We perform the time-resolved spectroscopy of the burst spectra described by a model consisting of thermal emission from the neutron star surface and a varying persistent emission component to study the evolution of burst parameters. The enhancement of the persistent emission during burst exposure is characterized by the scaling parameter f a, which reflects the increasing strength of the burst-disc interaction with burst intensity, likely driven by Poynting-Robertson drag. The spectral analysis of bursts estimate the average apparent blackbody emitting radius of the neutron star to lie within 1.5-3.5 km. The ignition depths computed from the burst parameters indicate short Type-I thermonuclear bursts from a mixed hydrogen-helium fuel layer.

Figures (8)

• Figure 1: Left panel:NuSTAR/FPMA light curve of the source 4U 1323-62 in the energy range 3-79 keV. Entire light curve for the $\sim$90 ks observation with the observed type-I thermonuclear bursts B1, B2, B3, B4, B5 and B6 where the start, end and duration times are noted in Table \ref{['tab:Burst_times']}; right panel: two light curves with 100 s bin size showing regular intensity dips with time interval- 27 ks from the start of observation (top), 40 ks within the time segment 40 ks and 80 ks from the beginning of observation (bottom), green double headed arrows show the separation between two consecutive intensity dips.
• Figure 2: Upper panel: 3-79 keV NuSTAR/FPMA lightcurve for B1, B2 and B3,respectively. Middle panel: 3-79 keV NuSTAR/FPMA lightcurve for B4, B5 and B6, respectively. The horizontal blue line shows the zero count level and the green dotted line for each plot shows the level for 25% of the peak count rate. Horizontal lines are needed to define $t_{\rm rise}, t_{\rm peak}$ and $t_{\rm decay}$. The vertical blue dash-dotted line corresponds to the peak count rate and brown dashed line marks the end of the exposure interval. Lower panel:NuSTAR/FPMA light curve of the six bursts (B1-B6) stacked together aligned at their starting point within 3-70 keV energy regime with a 10 s binsize; inset: 30-70 keV NuSTAR/FPMA light curve for B1-B6 with a binsize of 0.5 s, with all bursts aligned at their start.
• Figure 3: Upper Panel: Unfolded spectral plot for the best-fit data corresponding to the Model 4: const*Tbabs*edge*thcomp*diskbb is shown for the persistent NuSTAR spectrum of FPMA (blue) and FPMB (magenta) of the source 4U 1323-62 in the energy range 3-60 keV. Lower panel: Residual plot in 1 $\sigma$ error bar.
• Figure 4: Upper panel: Unfolded spectra of all the bursts are shown for the best-fit data obtained for the model Tbabs*(edge*nthcomp+bbodyrad) as indicated by the legend for colour at the top right corner of the plot for NuSTAR telescopic observation of FPMA in the energy range 3-20 keV for the source 4U 1323-62. Lower panel: Residual plot within 1 $\sigma$ error bar for the difference of data vs. model.
• Figure 5: Unfolded spectra (burst emission) of B1 using Upper panel: Best fit of Model 5: Tbabs*bbodyrad; Lower panel: Best fit of Model 7: Tbabs*($f_{\rm a}$*edge*nthcomp+bbodyrad), respectively for the NuSTAR/FPMA observation in the energy range 3-20 keV of the source 4U 1323-62. The lower plots within each plot shows the residual plot for the particular data and model within 1 $\sigma$ error bar. The time interval corresponding to Burst 1 segments S1-S5 are 25 s, 20 s, 20 s, 20 s and 40 s, respectively.